Unveiling shocks in planetary nebulae

TL;DR

This study uses HST images to map [O III]/Halpha ratios in planetary nebulae, identifying shock regions caused by outflows and expanding shells, and analyzing the effects of nebular density on emission ratios.

Contribution

It introduces a method to detect shocks in planetary nebulae using ratio maps from archival HST images, confirming the presence of shocks in specific morphological structures.

Findings

Shocks are confirmed in bow-shock structures and expanding shells.

Regions with depressed [O III]/Halpha ratios are linked to density-bounded PNe.

Enhanced ratios indicate shock-heated regions.

Abstract

The propagation of a shock wave into a medium is expected to heat the material beyond the shock, producing noticeable effects in intensity line ratios such as [O III]/Halpha. To investigate the occurrence of shocks in planetary nebulae (PNe), we have used all narrowband [O III] and Halpha images of PNe available in the HST archive to build their [O III]/Halpha ratio maps and to search for regions where this ratio is enhanced. Regions with enhanced [O III]/Halpha emission ratio can be ascribed to two different types of morphological structures: bow-shock structures produced by fast collimated outflows and thin skins enveloping expanding nebular shells. Both collimated outflows and expanding shells are therefore confirmed to generate shocks in PNe. We also find regions with depressed values of the [O III]/Halpha ratio which are found mostly around density bounded PNe, where the local contribution of [N II] emission into the F656N Halpha filter cannot be neglected.